Reliability Analysis Of Corroded Subsea Pipeline

Submarine pipeline is the most efficient and safe way\to transport fluids and gas over long distances for the oil and gas industry. The damage of submarine pipelines could cause huge economical loss and severely environmental pollution.The situation is severe especially in the harsh marine environment. Corrosion is one of the major factors which result in pipeline failure. It is vital to calculate the reliability of the corroded submarine pipelines and to predict the remaining life of pipelines. In order to maintain the safety of the corroded pipe, this paper has done the following research in the view of reliability analysis:1) In order to study the ultimate failure pressure of the corroded submarine pipe, numerical model of pipelines containing corrosion defects are established using FE software Abaqus. The The results of the numerical model is compared with the experimental results to testify the suitbility of the model. Based on FE analysis, research is conduct to study the influence of corrosion geometry and steel strength on the failure pressure of the pipelines. A new method for predicting the ultimate bending moment of the corroded submarine pipelines with single defect under a combination load of pressure and axial force has been developed. The ultimate bending moment of a plain pipe represents an upper limit for the failure moment of the pipe with a single corrosion defect, while the failure moment of a pipe with an infinite long corrosion represents the lower limit. The predicted failure moment can be calculated from these limits using the weight function method which accounts for the defect geometry. Then the weight function method is verified by the numerical calculation for a series of finite element models. This method can make a nice prediction for the corroded pipe with a single defect under internal pressure.2) In order to predict the ultimate pressure for infinite axial corroded pipes, this paper presents a simplified formula, while the modified formula is used for pipes subject to the localized rectangular corrosion. Meanwhile probabilistic method is used to conduct the upper and lower thresholds for the corroded pipes.3) In order to study the influence of the corrosion model on the failure probability of the corroded submarine pipe.This paper presents the failure function of the corroded submarine pipes based on the maximum corrosion depth. The power function corrosion model and the exponential function corrosion model are introduced in the calculation of the reliability of corroded pipes, compared with the commonly used linear model. According to the advantages and the reliability calculation, the power function model is recommended for reliability calculation. Meanwhile, the influence of the evaluation methods of remaining strength of corroded pipes on pipe reliability is studied.4) In order to study the seismic reliability of the suspended corroded submarine pipeline, a corrosion model of submarine pipe is established based on Markov chain. The simplified computation model is derived to calculate the axial displacement and stress under the earthquake. At the same time, Monte Carlo method is used to calculate the mean value and standard deviation. With a given sample, the seismic reliability is analyzed for a suspended corroded submarine pipe using the ultimate failure function of corroded submarine pipeline. Thus the reference to analyze seismic reliability is provided for the management of the submarine pipeline.5) In order to calculate the reliability of corroded subsea pipeline, a corrosion model, effect of loads that count in the service period of subsea pipeline are studied, including pressure, temperature load, bending moment, overlaying soil effect, earthquake effect and residual stress effect. The reliability of corroded submarine pipe under all these effects is calculated with Monte Carlo method based on numerical example, so that the change of failure probability is obtained. Meanwhile, the effect of these loads on the failure probability of corroded subsea pipe is quantitatively analyzed. Sensitivity analysis is conducted for parameters that are usually used in design such as operational pressure, pipe radius, thickness of pipe wall and et al, in the view of failure probability. The analysis provides references for reliability calculation of corroded subsea pipeline in practice.